Laramide exhumation of the Bighorn Mountains, Wyoming: An apatite (U-Th)/He thermochronology study

Abstract

We report (U-Th)/He apatite ages from the crystalline core of the Bighorn Mountains and compare the exhumation history derived from those ages with the exhumation history determined from sedimentary rocks in the adjacent Powder River and Bighorn basins. Our (U-Th)/He apatite ages range from 62 to 369 Ma and represent a pre-Laramide He partial retention zone that was deformed and uplifted at ca. 65 ′ 5 Ma. The geometry of the He partial retention zone indicates that the basement in the Bighorn Mountains is deformed into a doubly plunging anticline. The preservation of a pre-Laramide partial retention zone in the upper few hundred meters of Precambrian basement indicates that in general, the temperature at the Cambrian unconformity did not exceed the apatite He closure temperature. This is difficult to reconcile with evidence from adjacent basins for thick sequences of sedimentary rocks (3-4 km) prior to 65 Ma, and normal modern geothermal gradients. Either the range was never deeply buried (<2-3 km), its geothermal gradient has been low (<20 °C) since at least the Mesozoic, or our apatites have higher (U-Th)/He closure temperatures (∼80-90 °C) than those measured for other apatites.